In a typical raw sugar factory, the cane is prepared for grinding in the mills by revolving knifes. The prepared cane is then subjected to squeezing through heavy mill rollers where the juice is extracted. That juice contains both soluble and insoluble materials. The insoluble portion is primarily fiber with some soil particles which are mechanically removed by screening. The soluble portion is constituted mainly of various saccharides, predominantly sucrose, accompanied by a lesser amount of both organic and inorganic non-sugars. The percentage ratio of saccharides to total solubles, sugars and non-sugars, in solution is expressed as the Purity Index, which is typically in the range of 75 to 93. These non-sugars increase the solubility of sucrose, thus during the crystallization process, prevent by their presence, the crystallization of a portion of the sucrose fraction respective to the degree to which the solubility has been increased. Therefore, the objective is to purify the juice prior to crystallization. Purification, which is termed clarification, or defecation, is usually achieved by the addition of both heat and lime in various combinations. Unfortunately, no presently known method of clarification is capable of achieving the complete removal of the soluble non-sugars from the juice. In actuality, no more than a two point increase in Purity Index is ever obtained. Generally, sufficient lime is added to neutralize the organic acids present in the juice after which the temperature is raised to 210.degree. F. or above. This combined treatment forms a heavy precipitate of complex compositions containing insoluble lime salts, coagulated albumin and varying proportions of fats, waxes and gums. The precipitate is flocculent and carries with it most of the finely suspended material in the juice which has escaped the mechanical screening. The separation of the precipitate from the surrounding juice is almost universally accomplished by decantation in clarifiers, having a minimum retention time of approximately two hours. Under proper control, a clarified juice is obtained which is practically free of turbidity creating material and is almost transparent.
The clarified juice goes to the multiple effect evaporators without further treatment. The evaporators consist of a series of bodies; each succeeding body is boiled from the evaporation vapour of the preceding one and vapour from the last body being condensed in a contact condenser. From the evaporators, a syrup is obtained containing approximately 20% of the original water content of the clarified juice. The concentration of total soluble solids in the syrup is expressed in degree Brix and is usually in the range of 60.degree. to 70.degree., corresponding to a specific gravity of 1.28 to 1.35; the clarified juice being of 10.degree. to 13.degree. Brix with corresponding specific gravity varying between 1.03 and 1.05.
The crystallization of the syrup is performed in single effect vacuum pans under reduced pressure. The syrup is further evaporated until saturation. At this stage, crystals separate in the boiling mass or seed crystal is added to serve as nuclei for the crystals. The pan is charged with syrup intermittently as the water evaporates, and the sugar contained in the syrup is largely deposited upon the nuclei without the formation of additional crystals. When the pan is filled with crystal of appropriate size, the mixture of crystals and syrup is concentrated to a dense mass, "massecuite," which is then discharged from the pan. The massecuite is then purged in a high speed centrifugal having a cylindrical perforated metal basket, lined with wire cloth and perforated bronze screen. The sugar crystals are retained by the lining while the melter liquor, "molasses," is forced through the sugar layer and lining by centrifugal force. This molasses is crystallized again yielding a second grade sugar and molasses. Recrystallization of the molasses is repeated until such a lower molasses purity is obtained that it will no longer yield any more sugar by crystallization. That molasses is then discarded as Final Molasses which then contain such a high content of non-sugars to maintain its sucrose content in solution.
During the evaporation of the clarified juice to the syrup, a considerable quantity of non-sugars that were soluble in the clarified juice became insoluble in the syrup due to the loss of 80% of the juice's original water content. A small amount of these impurities deposit themselves upon the heating surface of the evaporator forming hard scale, but the major portion remain in suspension in the syrup and are entrained to the crystallization process. During the crystallization process, the syrup in the pan is subjected to further evaporation and while the sugar crystals are improving in size, the Purity Index of the syrup decreases as a result of the migration of the sugar fraction to the crystal surfaces. Thus, the soluble non-sugar content of the syrup increases, and as some of these non-sugars are high color contributors, the color of the syrup constituting the crystallization media tremendously increases towards approximately double its original colour value prior to crystallization. Therefore along the crystallization process, these insoluble impurities, precipitated out of the clarified juice during evaporation to syrup, are inevitably occluded within the various layers of the forming crystals and carry with them the high coloured mother liquor of decreasing purity. When this massecuite is purged in the centrifugal, a sugar of high apparent colour is obtained which has a well documented detrimental effect on refinability.
Furthermore, during purging, the portion of these insoluble non-sugars that were not occluded in the crystal, causes blockages of the channels existing between the crystals through which the mother liquor is supposed to be forced out. This results in more high coloured molasses being retained with the sugar further yielding a poor quality sugar which will create difficulties during the process of refining of the raw sugar to white consumer sugar, greatly increasing production costs.
Formerly, attempts were made to remove these insoluble impurities by settling and decantation in syrup-settling tanks having a retention time of six to twenty-four hours. Even then, the removal of sediments was largely incidental to these insoluble materials originally present in the juice not removed during the clarification process prior to evaporation. Furthermore, large modern factories do not have tank space to permit settling of syrup. Apart from the fact that the impurities precipitated or thrown out of solution during evaporation are mostly in the micron size and being slightly lighter than the syrup, they would therefore mostly remain in suspension while an insignificant amount would float. Various studies in literature verify this effect.
Various processes utilizing chemical additives, such as phosphates and lime in combination with moderate aeration, are utilized to float these impurities out of the syrup in low retention tanks or clarifier. These chemical processes suffer from various disadvantages. For example, when a calcium phosphate precipitate is formed in the syrup, they show high absorption capacity and will definitely attract the micron size precipitated impurities but unfortunately, their sizes are somewhat larger and therefore entrain with them a high proportion of the syrup itself and thus must be further subjected to another separation process to recover the syrup. Furthermore, these large calcium phosphate precipitates are friable and cannot be subjected to intensive aeration which can only be obtained by violent mechanical means; if subject to intensive aeration, a large portion of these precipitates are eroded and again somehow defeat the initial intent of their optimum utilization. Moreso, whenever chemicals are added to a sugar solution, it is impossible to ensure a complete interreaction between the various reagents, therefore a portion of these reagents remains in solution in the syrup, increasing the non-sugar concentration of the latter which result in lesser sugar recovery by crystallization due to the increase in sucrose solubility by the presence of these added non-sugars.
In Rundell et al U.S. Pat. No. 3,834,541, the inventors note that they have found the following steps necessary to achieve optimum flocculation of the suspended solids and efficient clarification of the liquor: a. forming a primary floc in the liquor; b. aerating the liquor containing the primary floc, with agitation; c. distributing an organic polymeric flocculant uniformly throughout the liquid phase of the aerated liquor to initiate the formation of a secondary floc therein; d. retaining the resulting mixture in a flocculator vessel with nonturbulent agitation preventing the segregation of the secondary floc from the liquor and allowing the secondary floc to grow; e. transferring the liquor, with minimum agitation, from the flocculator vessel to a separator vessel; f. allowing the secondary floc to segregate by flotation from the liquor in the separator vessel; and g. separately removing clarified liquor and flocculated solids from the separator vessel. "The aeration of the liquor containing the primary floc can be achieved by blowing in air under pressure, by venturi suction into a pipe through which the liquor is flowing, or by releasing air already present in the liquor, for example, mechanically or by heating the liquor. The air bubbles should be thoroughly broken up and mixed into the liquor, to produce a fine dispersion of air bubbles therein and ensure satisfactory subsequent aeration of the floc. This can be achieved for example, in the case of a melter liquor by using a centrifugal pump with an open impeller, operating at a tip speed of about 100 feet per second."
Some technologists, among many, have demonstrated that subjecting the syrup or any sugar media containing these precipitated non-sugars to centrifugal separators improves both the quality of the sugar as well as their recovery.
Unfortunately, these centrifugal separators, although fitted with self-cleaning devices, have to be dismantled at regular intervals for more adequate cleaning to maintain optimum efficiency. This situation calls for the installation of standby units to ensure continous operation and therefore has not been adopted by the sugar industry as a whole.